Insulin mediator stimulation of pyruvate dehydrogenase phosphatases

Metal dependent protein phosphatase PPM family in cardiac health and diseases

Protein phosphorylation and dephosphorylation is central to signal transduction in nearly every aspect of cellular function, including cardiovascular regulation and diseases. While protein kinases are often regarded as the molecular drivers in cellular signaling with high specificity and tight regulation, dephosphorylation mediated by protein phosphatases is also gaining increasing appreciation as an important part of the signal transduction network essential for the robustness, specificity and homeostasis of cell signaling. Metal dependent protein phosphatases (PPM, also known as protein phosphatases type 2C, PP2C) belong to a highly conserved family of protein phosphatases with unique biochemical and molecular features. Accumulating evidence also indicates important and specific functions of individual PPM isoform in signaling and cellular processes, including proliferation, senescence, apoptosis and metabolism. At the physiological level, abnormal PPM expression and activity have been implicated in major human diseases, including cancer, neurological and cardiovascular disorders. Finally, inhibitors for some of the PPM members have been developed as a potential therapeutic strategy for human diseases. In this review, we will focus on the background information about the biochemical and molecular features of major PPM family members, with emphasis on their demonstrated or potential roles in cardiac pathophysiology. The current challenge and potential directions for future investigations will also be highlighted.

Insulin-like and mimetic molecules from non-mammalian organisms: potential relevance for drug discovery

Insulin was first discovered in extracts of vertebrate pancreas during a focused search for a therapy for diabetes. Subsequent efforts to discover and isolate a similar active principle from yeast and plants driven by the hope to identify insulin-like/mimetic molecules with critical advantages in the pharmacokinetic profile and expenditure of production compared to authentic human insulin were not successful. As a consequence, it has generally been assumed that hormones evolved exclusively during course of the evolution of vertebrate endocrine organs, implying a rather recent origin. Concomitantly, the existence and physiological role of vertebrate hormones in lower multi- and unicellular eukaryotes have remained a rather controversial subject over decades, albeit there is some evidence that hormones and hormone-binding proteins resembling those of vertebrates are expressed in fungi and yeast. Past and recent findings on the existence of insulin-like and mimetic materials, such as the glucose tolerance factor, in lower eukaryotes, in particular Neurospora crassa and yeast, will be presented. These data provide further evidence for the provocative view that the evolutionary roots of the vertebrate endocrine system may be far more ancient than is generally believed and that the identification and characterisation of insulin-like/mimetic molecules from lower eukaryotes may be useful for future drug discovery efforts.

Proteomic analysis of mitochondrial biogenesis in cardiomyocytes differentiated from human induced pluripotent stem cells

Mitochondria play key roles in the differentiation and maturation of human cardiomyocytes (CMs). As human induced pluripotent stem cell-derived cardiomyocytes (hiPSC-CMs) hold potential in the treatment of heart diseases, we sought to identify key mitochondrial pathways and regulators, which may provide targets for improving cardiac differentiation and maturation. Proteomic analysis was performed on enriched mitochondrial protein extracts isolated from hiPSC-CMs differentiated from dermal fibroblasts (dFCM) and cardiac fibroblasts (cFCM) at time points between 12 and 115 days of differentiation, and from adult and neonatal mouse hearts. Mitochondrial proteins with a twofold change at time points up to 120 days relative to 12 days were subjected to ingenuity pathway analysis (IPA). The highest upregulation was in metabolic pathways for fatty acid oxidation (FAO), the tricarboxylic acid (TCA) cycle, oxidative phosphorylation (OXPHOS), and branched chain amino acid (BCAA) degradation. The top upstream regulators predicted to be activated were peroxisome proliferator-activated receptor γ coactivator 1 α (PGC1-α), the insulin receptor (IR), and the retinoblastoma protein (Rb1) transcriptional repressor. IPA and immunoblotting showed upregulation of the mitochondrial LonP1 protease-a regulator of mitochondrial proteostasis, energetics, and metabolism. LonP1 knockdown increased FAO in neonatal rat ventricular cardiomyocytes (nRVMs). Our results support the notion that LonP1 upregulation negatively regulates FAO in cardiomyocytes to calibrate the flux between glucose and fatty acid oxidation. We discuss potential mechanisms by which IR, Rb1, and LonP1 regulate the metabolic shift from glycolysis to OXPHOS and FAO. These newly identified factors and pathways may help in optimizing the maturation of iPSC-CMs.

Lactic acidosis treatment by nanomole level of spermidine in an animal model

Lactic acidosis occurs in a number of clinical conditions, e.g. in surgeries, orthotopic liver transplant, and anesthetic agent administration, which has deleterious effects on the patient's survival. The most rational therapy for these patients, the sodium bicarbonate administration, cannot prevent those accompanying deficiencies and may actually be harmful. In addition, tromethamine adjusts the blood pH, it does not affect the lactate accumulation. Therefore, discovery of a therapeutic agent is still a major unsolved problem. In this study, the rats were divided into different groups and lactic acidosis type B was induced in them. Then, the effect of different injection doses of spermidine (0-20nmol) on lactic acidosis was analyzed by measuring the lactate level and pH in the rat blood samples. The results showed that spermidine effectively and simultaneously inhibited the lactate and pyruvate accumulations, and also adjusted the pH of bloodstream. On the other hand, it has been shown (Damuni et al., 1984; Rahmatullah and Roche, 1988) that spermidine increases the activity of phosphatase, leading to prevention of lactate accumulation. The results indicate that administration of only nanomole level of spermidine may be the best treatment in the liver transplant and other patients suffering from lactic acidosis type B.

Herbal constituent sequoyitol improves hyperglycemia and glucose intolerance by targeting hepatocytes, adipocytes, and β-cells

The prevalence of insulin resistance and type 2 diabetes increases rapidly; however, treatments are limited. Various herbal extracts have been reported to reduce blood glucose in animals with either genetic or dietary type 2 diabetes; however, plant extracts are extremely complex, and leading compounds remain largely unknown. Here we show that 5-O-methyl-myo-inositol (also called sequoyitol), a herbal constituent, exerts antidiabetic effects in mice. Sequoyitol was chronically administrated into ob/ob mice either orally or subcutaneously. Both oral and subcutaneous administrations of sequoyitol decreased blood glucose, improved glucose intolerance, and enhanced insulin signaling in ob/ob mice. Sequoyitol directly enhanced insulin signaling, including phosphorylation of insulin receptor substrate-1 and Akt, in both HepG2 cells (derived from human hepatocytes) and 3T3-L1 adipocytes. In agreement, sequoyitol increased the ability of insulin to suppress glucose production in primary hepatocytes and to stimulate glucose uptake into primary adipocytes. Furthermore, sequoyitol improved insulin signaling in INS-1 cells (a rat β-cell line) and protected INS-1 cells from streptozotocin- or H₂O₂-induced injury. In mice with streptozotocin-induced β-cell deficiency, sequoyitol treatments increased plasma insulin levels and decreased hyperglycemia and glucose intolerance. These results indicate that sequoyitol, a natural, water-soluble small molecule, ameliorates hyperglycemia and glucose intolerance by increasing both insulin sensitivity and insulin secretion. Sequoyitol appears to directly target hepatocytes, adipocytes, and β-cells. Therefore, sequoyitol may serve as a new oral diabetes medication.

Synthetic inositol phosphoglycans related to GPI lack insulin-mimetic activity

Insulin signaling has been suggested, at least in part, to be affected by an insulin-mimetic species of low molecular weight. These inositol phosphoglycans (IPGs) are generated upon growth hormone/cytokine stimulation and control the activity of a multitude of insulin effector enzymes. The minimal structural requirements of IPGs for insulin-mimetic action have been debated. Two types of IPGs were suggested, and the IPG-A type resembles the core glycan of glycosylphosphatidylinositol (GPI)-anchors. In fact, purified GPI-anchors of lower eukaryotic origin have been shown to influence glucose homeostasis. To elucidate active IPGs, a collection of synthetic IPGs designed on the basis of previous reports of activity were tested for their insulin-mimetic activity. In vitro and ex vivo assays in rodent adipose tissue as well as in vivo analyses in mice were employed to test the synthetic IPGs. None of the IPGs we tested mimic insulin actions as determined by PKB/Akt phosphorylation and quantification of glucose transport and lipogenesis. Furthermore, none of the IPGs had any effect in in vivo insulin tolerance assays. In stark contrast to previous claims, we conclude that neither of the compounds tested is insulin-mimetic.

Age-related changes in the response of rat adipocytes to insulin: evidence for a critical role for inositol phosphoglycans and cAMP

Adipose tissue plays a pivotal role in ageing and longevity; many studies, both human and animal, have focussed on the effects of food limitation. Here we present a new model based on striking differences between two 'normal' inbred strains of albino Wistar rats the Charles River (CR) and Harlan Olac (HO) that have marked differences in age-related accumulation of fat and insulin-stimulated rates of glucose incorporation into lipid in the epididymal fat pads (EFP). The incorporation [U-(14)C]glucose into lipid by adipocytes showed that the CR group had a twofold higher basal rate of lipogenesis and a greater response to insulin in vitro, exceptionally, adipocytes from CR group maintained the high response to insulin to late adulthood while retaining the lower EFP weight/100 g body weight. Inositol phosphoglycan A-type (IPG-A), a putative insulin second messenger, was 3.5-fold higher and cAMP significantly lower per EFP in the CR versus HO groups. Plasma insulin levels were similar and plasma leptin higher in CR versus HO groups. The anomaly of a higher rate of lipogenesis and response to insulin and lower EFP weight in the CR group is interpreted as the resultant effect of a faster turnover of lipid and stimulating effect of leptin in raising fatty acid oxidation by muscle, potentially key to the lower accumulation of visceral fat. The metabolic profile of the CR strain provides a template that could be central to therapies that may lead to the lowering of both adipose and non-adipocyte lipid accumulation in humans in ageing.

The biological activity of structurally defined inositol glycans

BACKGROUND: The inositol glycans (IGs) are glycolipid-derived carbohydrates produced by insulin-sensitive cells in response to insulin treatment. IGs exhibit an array of insulin-like activities including stimulation of lipogenesis, glucose transport and glycogen synthesis, suggesting that they may be involved in insulin signal transduction. However, because the natural IGs are structurally heterogeneous and difficult to purify to homogeneity, an understanding of the relationship between structure and biological activity has relied principally on synthetic IGs of defined structure. DISCUSSION: This article briefly describes what is known about the role of IGs in signal transduction and reviews the specific biological activities of the structurally defined IGs synthesized and tested to date. CONCLUSION: A pharmacophore for IG activity begins to emerge from the reviewed data and the structural elements necessary for activity are summarized.

Reciprocal control of pyruvate dehydrogenase kinase and phosphatase by inositol phosphoglycans. Dynamic state set by "push-pull" system

Reversible phosphorylation of proteins regulates numerous aspects of cell function, and abnormal phosphorylation is causal in many diseases. Pyruvate dehydrogenase complex (PDC) is central to the regulation of glucose homeostasis. PDC exists in a dynamic equilibrium between de-phospho-(active) and phosphorylated (inactive) forms controlled by pyruvate dehydrogenase phosphatases (PDP1,2) and pyruvate dehydrogenase kinases (PDK1-4). In contrast to the reciprocal regulation of the phospho-/de-phospho cycle of PDC and at the level of expression of the isoforms of PDK and PDP regulated by hormones and diet, there is scant evidence for regulatory factors acting in vivo as reciprocal "on-off" switches. Here we show that the putative insulin mediator inositol phosphoglycan P-type (IPG-P) has a sigmoidal inhibitory action on PDK in addition to its known linear stimulation of PDP. Thus, at critical levels of IPG-P, this sigmoidal/linear model markedly enhances the switchover from the inactive to the active form of PDC, a "push-pull" system that, combined with the developmental and hormonal control of IPG-P, indicates their powerful regulatory function. The release of IPGs from cell membranes by insulin is significant in relation to diabetes. The chelation of IPGs with Mn2+ and Zn2+ suggests a role as "catalytic chelators" coordinating the traffic of metal ions in cells. Synthetic inositol hexosamine analogues are shown here to have a similar linear/sigmoidal reciprocal action on PDC exerting push-pull effects, suggesting their potential for treatment of metabolic disorders, including diabetes.

Insight into the role of inositol phosphoglycans in insulin response and the regulation of glucose and lipid metabolism illustrated by the response of adipocytes from two strains of rats

Differences in biochemical and hormone profiles between two strains of rats provide insights into the relationships between insulin response, inositol phosphoglycans and lipid metabolism in adipose tissue. The results suggest the apparent anomaly of a higher rate of lipogenesis and response to insulin with a lower fat pad weight in the Charles River vs. Harlan Olac group relates to: (i) enzyme pre-programming with IPG-A, (ii) faster turnover of lipid, (iii) effects of leptin and cAMP.

Preeclampsia, insulin signalling and immunological dysfunction: a fetal, maternal or placental disorder?

An inappropriate glycogen accumulation in preeclamptic placentas was described as secondary to biochemical alterations. Insulin resistance is widely accepted to be associated with preeclampsia, although its basis remain unclear. A family of putative insulin mediators, namely inositol phosphoglycans, were described to exert many insulin-like effects on lipid and glucose metabolism. A definite association between the P-type mediator (P-IPG) and preeclampsia was reported, being increased in placenta, urine, amniotic fluid and cord blood from human preeclamptic pregnancies. A strong link exists between insulin resistance and inflammation. Clear features of insulin resistance and systemic inflammatory activation were described in preeclampsia. It may be a consequence of the immunological dysfunction that occurs in preeclampsia that is temporized during sperm exposure and co-habitation which confuses the maternal immune network to perceive 'danger'. The over-expression of P-IPG during preeclampsia may be a counter-regulatory mechanism to insulin resistance since these molecules mimic insulin action. Besides, the lipidic form of P-IPG was reported to be similar to endotoxins, and may represent the 'danger signa'. We propose here a novel working theory on insulin resistance and preeclampsia.

Urinary excretion of inositol phosphoglycan P-type in gestational diabetes mellitus

OBJECTIVE

The mechanisms underlying insulin resistance during normal pregnancy, and its further exacerbation in pregnancies complicated by gestational diabetes mellitus (GDM), are generally unknown. Inositolphosphoglycan P-type (P-IPG), a putative second messenger of insulin, correlates with the degree of insulin resistance in diabetic subjects. An increase during normal pregnancy, in maternal and fetal compartments, has recently been reported.

METHODS

A cross-sectional study was carried out in 48 women with GDM and 23 healthy pregnant women. Urinary levels of P-IPG were assessed spectrophotometrically by the activation of pyruvate dehydrogenase phosphatase in urinary specimens and correlated with clinical parameters.

RESULTS

Urinary excretion of P-IPG was higher in GDM than in control women (312.1 +/- 151.0 vs. 210.6 +/- 82.7 nmol NADH/min/mg creatinine, P < 0.01) with values increasing throughout pregnancy in control subjects (r2 = 0.34, P < 0.01). P-IPG correlated with blood glucose levels (r(2) = 0.39, P < 0.01 for postprandial glycaemia and r2 = 0.18 P < 0.01 for mean glycaemia) and birthweight in the diabetic group (r2 = 0.14, P < 0.01).

CONCLUSIONS

Increased P-IPG urinary excretion occurs in GDM and positively correlates with blood glucose levels. P-IPG may play a role in maternal glycaemic control and, possibly, fetal growth in GDM.

Inositol Phosphoglycan P-Type in Preeclampsia

A state of insulin resistance has been demonstrated in active preeclampsia, and women with clinical evidence of insulin resistance are at higher risk to develop this syndrome during pregnancy. Recently, inositol phosphoglycan P-type, a putative second messenger of insulin action, has been implicated in the pathophysiology of preeclampsia and is increased in the placenta, amniotic fluid, and maternal urine of preeclamptic women compared with normal pregnant women. We report here a case–control study to assess the potential of urinary levels of inositol phosphoglycan P-type as a screening test for preeclampsia. Twenty-seven preeclamptic women and 47 healthy pregnant women were recruited. A polyclonal antibody-based ELISA was developed to detect levels of inositol phosphoglycan P-type in urine. Its content in urinary specimens was found to be 30-fold higher in preeclamptic subjects than control subjects (329.1±21.8 versus 9.2±1.5; P <0.001), with a higher level in all of the preeclamptic cases. For 6 women who developed preeclampsia, >1 gestational date sample of urine was available, and retrospective analysis showed a significant time-related increase of the urinary level of inositol phosphoglycan P-type ≤7 weeks before clinical diagnosis of preeclampsia. Urinary level of inositol phosphoglycan P-type increased after diagnosis indicating a possible pathophysiological threshold level and steeply decreased after delivery.

Insulin regulation of glutathione and contractile phenotype in diabetic rat ventricular myocytes

Cardiovascular complications of diabetes mellitus involve oxidative stress and profound changes in reduced glutathione (GSH), an essential tripeptide that controls many redox-sensitive cell functions. This study examined regulation of GSH by insulin to identify mechanisms controlling cardiac redox state and to define the functional impact of GSH depletion. GSH was measured by fluorescence microscopy in ventricular myocytes isolated from Sprague-Dawley rats made diabetic by streptozotocin, and video and confocal microscopy were used to measure mechanical properties and Ca(2+) transients, respectively. Spectrophotometric assays of tissue extracts were also done to measure the activities of enzymes that control GSH levels. Four weeks after injection of streptozotocin, mean GSH concentration ([GSH]) in isolated diabetic rat myocytes was approximately 36% less than in control, correlating with decreased activities of two major enzymes regulating GSH levels: glutathione reductase and gamma-glutamylcysteine synthetase. Treatment of diabetic rat myocytes with insulin normalized [GSH] after a delay of 3-4 h. A more rapid but transient upregulation of [GSH] occurred in myocytes treated with dichloroacetate, an activator of pyruvate dehydrogenase. Inhibitor experiments indicated that insulin normalized [GSH] via the pentose pathway and gamma-glutamylcysteine synthetase, although the basal activity of glucose-6-phosphate dehydrogenase was not different between diabetic and control hearts. Diabetic rat myocytes were characterized by significant mechanical dysfunction that correlated with diminished and prolonged Ca(2+) transients. This phenotype was reversed by in vitro treatment with insulin and also by exogenous GSH or N-acetylcysteine, a precursor of GSH. Our data suggest that insulin regulates GSH through pathways involving de novo GSH synthesis and reduction of its oxidized form. It is proposed that a key function of glucose metabolism in heart is to supply reducing equivalents required to maintain adequate GSH levels for the redox control of Ca(2+) handling proteins and contraction.

P-type inositol phosphoglycans in serum and amniotic fluid in active pre-eclampsia

OBJECTIVES

Abnormal secretion of P-type inositol phosphoglycans (IPG-P) has been described in maternal urine of pre-eclamptic women. The aim of this study was to determine the origin of production of IPG-P. We examined the IPG-P content of maternal and fetal serum, maternal urine and amniotic fluid in both normal pregnancy and pre-eclampsia.

DESIGN

Established extraction and bioactivity assay techniques were used to compare total IPG-P levels in serum samples, and a polyclonal-antibody-based ELISA to assay the amniotic fluid and urine samples in matched pairs of women.

SUBJECTS

Eleven women with pre-eclampsia requiring caesarean section (subjects), 11 pregnant women requiring elective caesarean section for reasons other than pre-eclampsia (controls).

RESULTS

Our data confirm the abnormal level of IPG-P in maternal urine during pre-eclampsia. Moreover, IPG-P levels were higher in umbilical sera than in maternal sera samples. Amniotic fluid as well as urine ELISA results were significantly higher in the pre-eclamptic group compared with normal controls. Total IPG-P bioactivity in serum did not vary between serum compartments in normal pregnancy. Uterine vein IPG-P levels were lower in pre-eclampsia when compared with normal pregnancy. A possible correlation was observed between urine and amniotic fluid levels in normal women. No correlation was observed between measured blood levels and those in urine and amniotic fluid.

CONCLUSIONS

It is hypothesized that steady state equilibrium of IPG-P in serum in normal pregnancy is disrupted in pre-eclampsia. Additionally, an abnormal IPG-P sub-fraction, detectable in urine and amniotic fluid, may be present and involved in the pathophysiology of the syndrome, although sites of production of this abnormal form remain unclear.

Insulin resistance in human preeclamptic placenta is mediated by serine phosphorylation of insulin receptor substrate-1 and -2

CONTEXT

Preeclampsia is a severe complication of human pregnancy often associated with maternal risk factors. Insulin resistance represents a major risk for developing preeclampsia during pregnancy.

OBJECTIVE

A putative second messenger of insulin, inositol phosphoglycan P type (P-IPG), was previously shown to be highly increased during active preeclampsia. Its association with insulin resistance was investigated.

DESIGN AND SETTING

A cross-sectional study was carried out in a referral center.

PATIENTS

Nine preeclamptic (PE) and 18 healthy women were recruited and matched for maternal age, body mass index, parity, and ethnicity in a 1:2 ratio. Placental specimens were collected immediately after delivery.

INTERVENTION

Placental tissue was incubated with insulin and P-IPG production assessed. Insulin signaling proteins were subsequently studied by immunoblotting.

RESULTS

P-IPG extracted from human term placentas upon incubation with insulin was found to be far lower in those with preeclampsia than controls (P < 0.001). Immunoblotting studies revealed serine phosphorylation of insulin receptor substrate-1 and -2 in PE placentas (P < 0.001) with downstream impairment of insulin signaling. The activation of the p85 regulatory subunit of phosphatidylinositol 3- kinase was markedly decreased in PE samples (P < 0.001).

CONCLUSIONS

These findings highlight the importance of P-IPG in active preeclampsia and demonstrate a substantially different response to the insulin stimulus of human PE placentas. Acquired alterations in activation of proteins involved in insulin signaling may play a role in the complex pathogenesis of preeclampsia, probably as a consequence of the immunological dysfunction that occurs in this syndrome. These results seem to confirm an insulin-resistant state in PE placenta and shed a different light on its role in the pathogenesis of this disease with potential therapeutic implications.

A fluorescent inositol phosphate glycan stimulates lipogenesis in rat adipocytes by extracellular activation alone

The chemical synthesis of 2,6-dideoxy-2-amino-6-mercaptoglucopyranosyl-(alpha1-6)-myo-inositol 1,2-cyclic phosphate and its conjugation with a lucifer yellow derivative are reported. The resulting fluorescent IPG analogue was able to stimulate lipogenesis in rat adipocytes despite the fact that it was not internalized into the cell. The results demonstrate that internalization of the IPG is not required for manifestation of its insulin-like effects.

Isolation, structure, synthesis, and bioactivity of a novel putative insulin mediator. A galactosamine chiro-inositol pseudo-disaccharide Mn2+ chelate with insulin-like activity

We isolated from beef liver a putative insulin mediator termed INS-2, 1. Its structure was determined to be a novel inositol glycan pseudo-disaccharide Mn(2+) chelate containing D-chiro-inositol 2a (as pinitol) and galactosamine. Purification methods were scaled up from those previously reported to isolate an inositol glycan with similar composition from rat liver.(1) Structure of the beef liver glycan was determined by degradative chemistry and 2D NMR spectroscopy and confirmed by chemical synthesis. Its structure is 4-O-(2-amino-2-deoxy-beta-D-galactopyranosyl)-3-O-methyl-D-chiro-inositol 1 (INS-2, Figure 1). Its role as an insulin mimetic was demonstrated by its action in vivo to decrease elevated blood glucose injected to low-dose streptozotocin diabetic rats in a stereospecific and dose-dependent manner. The pseudo-disaccharide also stimulated [(14)C]glucose incorporation into [(14)C]glycogen in a dose-dependent manner in H4IIE hepatoma cells in the presence of insulin, thus enhancing insulin action. Only when chelated to Mn(2+) did it activate pyruvate dehydrogenase phosphatase in vitro in a dose-dependent manner. To our knowledge, this is the first example of a beta-1,4-linked inositol glycan consisting of D-chiro-inositol and galactosamine isolated from animal tissues with insulin mimetic actions.

Differential modulation of glucose, lactate, and pyruvate oxidation by insulin and dichloroacetate in the rat heart

Despite the fact that lactate and pyruvate are potential substrates for energy production in vivo, our understanding of the control and regulation of carbohydrate metabolism is based principally on studies where glucose is the only available carbohydrate. Therefore, the purpose of this study was to determine the contributions of lactate, pyruvate, and glucose to energy production in the isolated, perfused rat heart over a range of insulin concentrations and after activation of pyruvate dehydrogenase with dichloroacetate (DCA). Hearts were perfused with physiological concentrations of [1-13C]glucose, [U-13C]lactate, [2-13C]pyruvate, and unlabeled palmitate for 45 min. Hearts were freeze clamped, and 13C NMR glutamate isotopomer analysis was performed on tissue extracts. Glucose, lactate, and pyruvate all contributed significantly to myocardial energy production; however, in the absence of insulin, glucose contributed only 25-30% of total pyruvate oxidation. Even under conditions where carbohydrates represented >95% of substrate entering the tricarboxylic acid (TCA) cycle, we found that glucose contributed at most 50-60% of total carbohydrate oxidation. Despite being present at only 0.1 mM, pyruvate contributed between approximately 10% and 30% of total acetyl-CoA entry into the TCA cycle. We also found that insulin and DCA not only increased glucose oxidation but also exogenous pyruvate oxidation; however, lactate oxidation was not increased. The differential effects of insulin and DCA on pyruvate and lactate oxidation provide further evidence for compartmentation of cardiac carbohydrate metabolism. These results may have important implications for understanding the mechanisms underlying the beneficial effects of increasing cardiac carbohydrate metabolism.

Regulation of glucose utilization and lipogenesis in adipose tissue of diabetic and fat fed animals: effects of insulin and manganese

In order to evaluate the modulatory effects of manganese, high fat diet fed and alloxan diabetic rats were taken and the changes in the glucose oxidation, glycerol release and effects of manganese on these parameters were measured from adipose tissue. An insulin-mimetic effect of manganese was observed in the adipose tissue in the controls and an additive effect of insulin and manganese on glucose oxidation was seen when Mn2+ was added in vitro. The flux of glucose through the pentose phosphate pathway and glycolysis was significantly decreased in high fat fed animals. Although the in vitro addition of Mn2+ was additive with insulin when 14CO2 was measured from control animals, it was found neither in young diabetic animals (6-8 weeks old) nor in the old (16 weeks old). Both insulin and manganese caused an increased oxidation of carbon-1 of glucose and an increase of its incorporation into 14C-lipids in the young control animals; the additive effect of insulin and manganese suggests separate site of action. This effect was decreased in fat fed animals, diabetic animals and old animals. Manganese alone was found to decrease glycerol in both the control and diabetic adipose tissue in in vitro incubations. The results of the effects of glucose oxidation, lipogenesis, and glycerol release in adipose tissue of control and diabetic animals of different ages are presented together with the effect of manganese on adipose tissue from high fat milk diet fed animals.

Activation and mitochondrial translocation of protein kinase Cdelta are necessary for insulin stimulation of pyruvate dehydrogenase complex activity in muscle and liver cells

In L6 skeletal muscle cells and immortalized hepatocytes, insulin induced a 2-fold increase in the activity of the pyruvate dehydrogenase (PDH) complex. This effect was almost completely blocked by the protein kinase C (PKC) delta inhibitor Rottlerin and by PKCdelta antisense oligonucleotides. At variance, overexpression of wild-type PKCdelta or of an active PKCdelta mutant induced PDH complex activity in both L6 and liver cells. Insulin stimulation of the activity of the PDH complex was accompanied by a 2.5-fold increase in PDH phosphatases 1 and 2 (PDP1/2) activity with no change in the activity of PDH kinase. PKCdelta antisense blocked insulin activation of PDP1/2, the same as with PDH. In insulin-exposed cells, PDP1/2 activation was paralleled by activation and mitochondrial translocation of PKCdelta, as revealed by cell subfractionation and confocal microscopy studies. The mitochondrial translocation of PKCdelta, like its activation, was prevented by Rottlerin. In extracts from insulin-stimulated cells, PKCdelta co-precipitated with PDP1/2. PKCdelta also bound to PDP1/2 in overlay blots, suggesting that direct PKCdelta-PDP interaction may occur in vivo as well. In intact cells, insulin exposure determined PDP1/2 phosphorylation, which was specifically prevented by PKCdelta antisense. PKCdelta also phosphorylated PDP in vitro, followed by PDP1/2 activation. Thus, in muscle and liver cells, insulin causes activation and mitochondrial translocation of PKCdelta, accompanied by PDP phosphorylation and activation. These events are necessary for insulin activation of the PDH complex in these cells.

In obese individuals dexfenfluramine corrects molecular derangements reflecting insulin resistance

BACKGROUND

Circulating lymphocytes of obese individuals with and without type 2 diabetes have derangements of pyruvate dehydrogenase (PDH) that are described as reflecting a disorder underlying systemic insulin resistance, namely basal activity below normal and, in vitro, unresponsiveness to insulin at 33 pmol/l and activation at 330 pmol/l instead of activation and inhibition as in controls.

OBJECTIVE

To explore whether the above enzyme derangements are overcome in obese individuals on dexfenfluramine treatment, known to improve poor peripheral insulin sensitivity.

METHODS

Fifteen obese diabetic patients and 15 age-matched euglycaemic obese subjects with normal glucose tolerance were enrolled for a trial composed of two 21-day periods; in the first (D-21-D0), participants received a placebo, and in the second (D0-D21), dexfenfluramine (30 mg/day). At D-21, D0 and D21 participants were evaluated for weight, BMI, fasting glycaemia (FG), fasting insulinaemia (FI), fasting insulin resistance index (FIRI), area under the glycaemic (G-AUC) and insulinaemic (I-AUC) curves from an OGT test, and for PDH activity assayed in their circulating lymphocytes before (basal activity) and after incubation with 33 or 330 pmol/l insulin. At D2, basal PDH activity and clinical parameters were assayed.

RESULTS

In both groups of participants at D0 all parameters tested were constant with respect to D-21; at D2, only basal PDH activity rose significantly; at D21, basal and insulin stimulated PDH activities were normalized and weight decreased significantly, as did FG, FI, FIRI and G-AUC in the diabetic, and FI, FIRI, G-AUC and I-AUC in the non-diabetic participants.

CONCLUSION

In obese, non-diabetic and diabetic individuals on dexfenfluramine treatment, amelioration of clinical parameters and indexes of poor insulin sensitivity of blood glucose homeostasis are preceded by correction, in their circulating lymphocytes, of PDH derangements described as reflecting a disorder underlying insulin resistance.

Inositol phosphoglycans and the regulation of the secretion of leptin: in vitro effects on leptin release from adipocytes and the relationship to obesity

The ratio of two families of inositol phosphoglycans (IPG-A:IPG-P), insulin second messengers, is raised in non-insulin-dependent diabetes mellitus (NIDDM) and obesity. It is shown here that IPG A type inhibits leptin release from adipocytes, contrasting with the action of insulin (stimulation) and IPG P type (no effect). The significance of inhibitory effects of IPG A type on leptin release is important in relation to obesity and NIDDM in view of the action of leptin in promoting Lep expression and fat oxidation in muscle, in addition to its effects on satiety. Energy conservation and oxidation via interorgan regulation by leptin could be compromised by a rise in the IPG-A:IPG-P ratio.

Inositol phosphoglycans and signal transduction systems in pregnancy in preeclampsia and diabetes: evidence for a significant regulatory role in preeclampsia at placental and systemic levels

Measurements have been made of the urinary content of inositol phosphoglycans IPG P-type and IPG A-type, putative insulin second messengers, in preeclampsia, in type I insulin-treated diabetic pregnant women and their matched control subjects, and nonpregnant women of child-bearing age. The content of IPG P-type and IPG A-type was also measured in the placenta from preeclamptic patients and from normal pregnancies. Pregnancy was associated with an increase, approximately twofold, in urinary output of IPG-P-type relative to nonpregnant controls (P<0.01). The 24-h output of IPG P-type in urine in preeclamptic women was significantly higher (2- to 3-fold) than in pregnant control subjects matched for age, parity, and stage of gestation (P<0.02). In contrast, insulin-dependent diabetic pregnant women did not show any significant change in urinary output of IPG P-type or IPG A-type relative to pregnant control subjects. Evidence for a possible relationship and correlation between the urinary excretion of IPG P-type and markers of preeclampsia, including proteinuria (r = 0.720, P<0.01), plasma aspartate transaminase (r = 0.658, P<0.05), and platelet counts (r = 0.613, P<0.05) is presented. A high yield of IPG P-type was extracted from human placenta, in preeclampsia some 3-fold higher (P = 0.03) than the normal value, whereas no IPG A-type (with lipogenic-stimulating activity) was found. Low concentrations of placental IPG A-type were detected relative to IPG P-type using assay systems dependent upon the effect of this mediator on cAMP-dependent protein kinase or on a proliferation assay using thymidine incorporation into DNA of EGFR T17 fibroblasts. It is postulated that the high urinary excretion IPG P-type in preeclampsia reflects high placental levels and relates to the accumulation of glycogen in the placenta. The paracrine effects of placental IPG P-type (stimulation off other endocrine glands and/or endothelial cells) could contribute to the pathogenesis of the maternal syndrome. A possible theoretical link between elevated placental IPG P-type and apoptosis is proposed.

Inositol phosphoglycans in diabetes and obesity: urinary levels of IPG A-type and IPG P-type, and relationship to pathophysiological changes

Measurements have been made, in adult male diabetic patients and control subjects, of the urinary content of inositol phosphoglycans (IPGs), the IPG A-type and IPG P-type forms, which, among other actions, regulate pathways of glucose utilization, lipogenesis, triglyceride formation, and pyruvate dehydrogenase (PDH) activity. Urine samples from the entire diabetic group showed a 2- to 3-fold increase in IPG A-type, and a fall in the IPG P-type:IPG A-type ratio relative to the control group. Subdivision of the diabetic patients into lean IDDM and obese NIDDM groups revealed significant differences in the IPG P-type:IPG A-type ratio between these groups, this ratio decreasing with increases in the body mass index (BMI). Analysis of the relationships among IPGs and HbA1, blood pressure, and BMI indicated that a fall in the IPG P-type:IPG A-type ratio correlated with a rise in the HbA1 (indicative of impaired glycemic control), with increased systolic blood pressure and increased obesity, all factors linked to Syndrome X. There was a parallism between the profile of the IPG P-type:IPG A-type ratio and the well-established pattern of insulin resistance and BMI. In vitro studies of the effects of alterations in the IPG P-type:IPG A-type ratio on the activation of the pyruvate dehydrogenase complex (PDH complex) at the PDH phosphatase reaction demonstrated that IPG A-type forms antagonized the stimulation of the PDH phosphatase by IPG P-type forms, thus having a negative effect on the conversion of PDH to the active, dephosphorylated, form. This observation could provide a mechanism whereby the shifts in the IPG P-type:IPG A-type ratio reported above could change the metabolic pattern from one directed to glucose oxidation to one more directed toward energy conservation and lipid storage.

Derangements of pyruvate dehydrogenase in circulating lymphocytes of NIDDM patients and their healthy offspring

Pyruvate dehydrogenase (PDH) is poorly active in circulating lymphocytes of NIDDM patients; in vitro, it is unresponsive to insulin at 5 microU/ml and activated at 50 microU/ml, instead of activated and inhibited as in healthy controls. This study examines whether healthy offspring of NIDDM patients with a family history for this disease have these alterations. Twenty seven healthy offspring (23+/-10 yr, median 18 yr) and their parents (13 diabetic with a family history for NIDDM and 11 healthy without this history) were enrolled. Twenty healthy individuals without the history and matched for age and gender with the offspring served as controls. Minimum levels for enzyme activity before and after cell stimulation with insulin at 5 microU/ml were computed for a 95% CI with no more than 5% of the controls excluded. Increased or unvaried enzyme activity in response to insulin at 50 microU/ml was defined as abnormal. All NIDDM parents and 11/27 offspring had below normal enzyme activity and defective and reversed enzyme response to insulin at 5 and 50 microU/ml; three offspring had altered enzyme response to insulin at both concentrations, four to insulin at 5 microU/ml, three to insulin at 50 microU/ml and six, together with the healthy parents, had no alterations. We conclude that in healthy individuals a family history for NIDDM is frequently signaled, irrespective of age, by molecular derangements, with an apparent genetic background, in their circulating lymphocytes.

Insulin stimulates pyruvate dehydrogenase and protects human ventricular cardiomyocytes from simulated ischemia

Impaired myocardial metabolism after cardioplegic arrest results in persistent anaerobic lactate production. Insulin may protect the heart from ischemia and reperfusion by enhancing myocardial metabolic recovery. However, the stimulation of glycolysis during ischemia may be detrimental because of an accumulation of metabolic end-products. We examined the effect of insulin on quiescent human ventricular cardiomyocytes subjected to simulated cardioplegic ischemia and reperfusion.

METHODS

Primary cardiomyocyte cultures were established from patients undergoing corrective repair of tetralogy of Fallot. Cells were exposed to varying concentrations of glucose and insulin during 30 minutes of stabilization in 10 mL of phosphate-buffered saline solution. Ischemia was simulated by exposing the cells to a low volume (1.5 mL) of deoxygenated phosphate-buffered saline solution for 90 minutes followed by 30 minutes of simulated reperfusion in 10 mL of normoxic phosphate-buffered saline solution. Cell viability was assessed by trypan blue exclusion. The activity of mitochondrial pyruvate dehydrogenase was measured in 3 states: stabilization, ischemia, and reperfusion. In addition intracellular lactate, adenine nucleotides, extracellular lactate, pyruvate, and acid release were measured.

RESULTS

Higher ambient glucose concentrations resulted in greater cellular injury although insulin-treated cells displayed less injury after ischemia and reperfusion. Insulin increased the pyruvate dehydrogenase activity by 31% in cardiomyocytes and reduced extracellular lactate production by 40%. Intracellular adenosine triphosphate was improved by 75% in cells exposed to high glucose concentrations in the presence of insulin.

CONCLUSIONS

Insulin protected human ventricular cardiomyocytes from ischemia and reperfusion. This protection may be due to a stimulation of pyruvate dehydrogenase activity which resulted in improved aerobic metabolism.

Molecular effects of sulphonylurea agents in circulating lymphocytes of patients with non-insulin-dependent diabetes mellitus

AIMS

In circulating lymphocytes of NIDDM patients pyruvate dehydrogenase (PDH), the major determinant in glucose consumption through oxidative pathways, is poorly active. The aim of this study is to examine whether sulphonylurea drug treatment revives PDH activity in circulating lymphocytes from NIDDM patients.

METHODS

Twenty normal-weight individuals with NIDDM were enrolled in this study. They had maintained their glycaemic levels close to normal by means of a restricted diet that had no longer been successful in the proceeding 2 months. The treatment protocol consisted in 160 mg gliclazide daily for 5 weeks. Twenty healthy subjects, matched for age, body mass index and gender, were enrolled as a control group. Patients, before and after treatment, as well as controls were tested for PDH activity in their circulating lymphocytes. Nine other untreated patients and nine healthy subjects, with the above mentioned characteristics, were recruited for the assay of PDH activity in their circulating lymphocytes before and after exposure, in vitro, to gliclazide, to insulin, and to gliclazide and insulin in combination.

RESULTS

In gliclazide-treated NIDDM patients, PDH activity in circulating lymphocytes recovered. In vitro, in circulating lymphocytes of untreated patients and controls insulin at 5 microU ml(-1) was ineffective and highly effective, respectively, in raising enzyme activity; gliclazide at 10 ng ml(-1) was ineffective on PDH in both groups, but in combination with insulin at 5 microU ml(-1) in both groups PDH was as active as in cells of controls exposed to insulin only. In cells of controls, gliclazide alone at 25-50 ng ml(-1) caused enzyme activation, whereas above 50 ng ml(-1) it caused inhibition; in cells of patients below 50 ng ml(-1) it had no effects, but at 50 ng ml(-1) and above raised enzyme activity to the basal level of controls.

CONCLUSIONS

This study suggests that free gliclazide concentrations determine recovery of PDH activity in circulating lymphocytes of treated patients through drug-mediated enhanced insulin control over PDH or through the drug alone.

G proteins and regulation of pyruvate dehydrogenase activity by insulin in human circulating lymphocytes

Pertussis toxin (PT) catalyzes ADP-ribosylation of G protein alpha subunits, thus preventing their role as transducers of external signals targeting metabolic pathways. In vitro, in human circulating lymphocytes insulin at physiological concentrations (5 microU/ml) determines sharp activation of pyruvate dehydrogenase (PDH), the rate limiting enzyme in glucose oxidative breakdown. This study evaluates whether the above-described effects of insulin over PDH are mediated through G proteins. Human circulating lymphocytes (six samples from different donors) were exposed to insulin (5 microU/ml), PT (1-2 micrograms/ml) or PT-9K, a mutated PT void of catalytic activity (1-10 micrograms/ml), and to insulin in combination with the two toxins, and then assessed for PDH activity. Plasma membranes from cells incubated with and without PT or PT-9K were subjected to ADP-ribosylation in the presence of [32P] NAD+ and activated PT. In circulating lymphocytes exposed to PT alone, or in combination with insulin, PDH activity falls significantly below basal values (P < 0.001); PT-9K instead has no effect on basal or on insulin-stimulated PDH activity. ADP-ribosylation of a plasma membrane component with apparent molecular mass (42 kDa) comparable to that of the Gi (inhibitory) protein alpha subunit takes place in cells exposed to PT but not in those exposed to PT-9K. In human circulating lymphocytes Gi proteins or Gi protein-like components appear to be involved in preserving basal PDH activity as well as in the mechanism by which insulin exerts its control over PDH.

Isolation and partial characterisation of insulin-mimetic inositol phosphoglycans from human liver

Extracts of human liver were found to contain activities which copurified and coeluted with the two major subtypes of mediators (type A and type P) isolated from insulin-stimulated rat liver. The putative type A mediator from human liver inhibited cAMP-dependent protein kinase from bovine heart, decreased phosphoenolypyruvate carboxykinase mRNA levels in rat hepatoma cells, and stimulated lipogenesis in rat adipocytes. The putative type P mediator stimulated bovine heart pyruvate dehydrogenase phosphatase. Both fractions were able to stimulate proliferation of EGFR T17 fibroblasts and the type A was able to support growth in organotypic cultures of chicken embryo cochleovestibular ganglia. Both activities were resistant to Pronase treatment and the presence of carbohydrates, phosphate, and free-amino groups were confirmed in the two fractions. These properties are consistent with the structure/ function characteristics of the type A and P inositolphosphoglycans (IPG) previously characterized from rat liver. Further, the ability of the human-derived mediators to interact with rat adipocytes and bovine-derived metabolic enzymes suggests similarity in structure between the mediators purified from different species. Galactose oxidase-susceptible membrane-associated glycosylphosphatidylinositols (GPI) have been proposed to be the precursors of IPG. GPI was purified from human liver membranes followed by treatment with galactose oxidase and reduction with NaB3H4. Serial t.l.c. revealed three radiolabeled bands which comigrated with the putative GPI precursors found in rat liver. These galactose-oxidase-reactive lipidic compounds, however, were only partially susceptible to hydrolysis with phosphatidylinositol-specific phospholipase C from Bacillus thuringiensis and were resistant to glycosylphosphatidylinositol-specific phospholipase C from Trypanosoma brucei. These data indicate that IPG molecules with insulin-like biological activities are present in human liver.

Characterization of a nucleotide stimulated aspartic proteinase in rat liver plasma membranes

Inositol phosphoglycan molecules are believed to mediate multiple intracellular actions of insulin. They are released from plasma membranes in response to insulin binding and are transported into the cell. Release of insulin mediators is stimulated by MnATP and MgATP and is inhibited by p-aminobenzamidine. Inositol phosphoglycan mediators may be released by a poorly characterized mechanism requiring proteolytic cleavage of an attached protein from the mediator and phospholipase cleavage of the mediator from its membrane anchor. We examined rat liver plasma membranes for proteinase activity stimulated by insulin and MnATP. Although we could not demonstrate insulin stimulation, we have found and characterized a nucleotide-stimulated aspartic proteinase bound to rat liver plasma membranes. We also detected and separated a soluble activating factor for the proteinase. The activating factor appears to be a protein with M(r) approximately 70 kDa.

Inhibition of hepatocytic autophagy by okadaic acid and other protein phosphatase inhibitors

Autophagy, measured as the sequestration of electroinjected [3H]raffinose or endogenous lactate dehydrogenase, was inhibited in isolated rat hepatocytes by the protein phosphatase inhibitors okadaic acid, calyculin A and microcystin-LR. Okadaic acid, the most potent inhibitor, suppressed autophagy almost completely at 15 nM, suggesting inhibition of a protein phosphatase of type 2A. Okadaic acid had no effect on ATP levels, protein synthesis or cellular viability at this concentration, but caused a disruption of the hepatocytic cytoskeleton and a consequent reduction in organelle sedimentability, potentially interfering with the autophagy assay unless the necessary precautions are taken. Lysosomal (propylamine-sensitive) degradation of endogenous protein was inhibited by okadaic acid, whereas non-lysosomal (propylamine-resistant) degradation was unaffected. The autophagy-inhibitory effect of okadaic acid was not affected by inhibitors of cAMP-dependent protein kinase or protein kinase C (H-7, H-89, calphostin C) but eliminated by the non-specific inhibitor K-252a and its analogues (KT-5720, KT-5823, KT-5926) and by KN-62, a specific inhibitor of Ca2+/calmodulin-dependent protein kinase II. Protein phosphorylation by this kinase would thus seem to play a role in regulation of the autophagic-lysosomal degradation pathway.

chiro-inositol deficiency and insulin resistance: a comparison of the chiro-inositol- and the myo-inositol-containing insulin mediators isolated from urine, hemodialysate, and muscle of control and type II diabetic subjects

chiro- and myo-Inositols are major components of the two inositol phosphoglycan mediators of insulin action. Previous work in this laboratory has shown hypo-chiro-inositoluria in type II diabetic subjects and decreased chiro-inositol in mediator prepared from skeletal-muscle biopsies of Pima Indian diabetic subjects together with increased myo-inositol concentrations. Because mediator bioactivity was not previously examined, we decided to isolate the two types of insulin mediator from hemodialysate, urine, and autopsy muscle to investigate their bioactivity in control and type II diabetic subjects. Human mediator fractions were isolated at pH 2.0 and pH 1.3 from hemodialysate, urine, and autopsy muscle of type II diabetic subjects and nondiabetic control subjects. Mediators were assayed for bioactivity, and the relative chiro-inositol/myo-inositol concentration ratio was determined for the mediator pH 2.0 samples by using HPLC or GC/MS. Regardless of source, the chiro-inositol-containing mediator pH 2.0 fractions from type II diabetic subjects were markedly less active than those from controls (50% or less) (P < 0.05). In addition, the chiro-inositol/myo-inositol ratio in samples from type II subjects was significantly reduced (1/3-1/9) compared with controls (P < 0.05 for hemodialysate and P < 0.01 for muscle samples). In contrast, no difference in bioactivity was seen in myo-inositol-containing mediator pH 1.3 samples isolated from the same type II diabetic and control subjects. In type II diabetes there is a generalized deficiency of chiro-inositol mediator in the body in terms of both decreased chiro-inositol mediator (pH 2.0) bioactivity and chiro-inositol content.